KW11-L Line Time Clock

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The KW11-L Line Time Clock is a UNIBUS device option which provides clock interrupts at a rate of 50 or 60 Hz, driven from the AC power provided to the CPU's power supply.

The programming interface is extremely simple; there is one register, with two bits; one to enable interrupts, and one which is set for every clock cycle (and causes an interrupt when that happens, if the other bit is set).

Physically, it is an extended-length single-height board, which plugs into a dedicated slot in the CPU's backplane.

Only the PDP-11/20, PDP-11/40, PDP-11/45 and PDP-11/70 support it. On later machines (e.g. the PDP-11/04 and PDP-11/34), the functionality was included in the DL11-W.